In a case of a laser diode, even if the driving current is kept at a constant level, light output fluctuates due to influences by changes of temperature and also an output image from an image forming apparatus becomes unstable, so that it is necessary to maintain the light output at a constant level by monitoring the light output at a specified timing. For this purpose, generally in an image forming apparatus to write image data on a photoconductive element using a laser diode, feedback control is executed by detecting light output from the laser diode.
FIG. 6 is a schematic diagram of an LD-driving board in a conventional type of image forming apparatus of a laser beam generating circuit in which, and as shown in this figure, on an LD-driving board 601 there are provided a semiconductor laser diode unit 602 in which a laser diode and a photo diode to detect strength of a laser beam generated by the laser diode have been integrated into a unit, an amplifier 603 to amplify an output from the photodiode, a driving current switching circuit 604 to drive the semiconductor laser diode unit 602, and a variable constant current supply unit 605 to supply a driving current for driving the semiconductor laser diode unit 602. In an image forming apparatus using a LD driving board having the configuration as described above, feedback control is executed via a D/A convertor 610 and the variable constant current supply unit 605 by monitoring light output from a laser diode with the amplifier 603, a D/A convertor 608, and a comparator 609 under the control by a CPU 607 on an engine board 606 for controlling printing operations.
Also to an image forming apparatus such as a printer, a demand for intermediate tone expression is high, so that a technique for using density expression making use of a plurality of adjacent dots is employed for intermediate tone expression (dither method). In the dither method, however, it is possible to provide a dummied intermediate tone, but the resolution becomes lower, so that development of a 1-dot multivalued image forming apparatus which can express 1 dot in multiple stages has been strongly desired.
With a conventional type of image forming apparatus, however, feedback control to maintain light output from a laser diode at a constant level is executed by a CPU on an engine board for controlling printing operations of the image forming apparatus, thus a large work load is put on the CPU during execution of this control, which disables printing operation control.
Also in a conventional type of image forming apparatus, to express 1 dot in multiple stages, such methods as changing the lighting time for each dot in a laser diode or changing exposure power are conceivable, but as minute and precise control over the lighting time is required for minutely changing the lighting time for each dot, the video signal generating mechanism becomes complex, which in turn results in increase of apparatus cost, and in addition a demand for noise resistance in the transmission path becomes more strict because of the necessity to transfer the signal at a higher speed. Also when changing the exposure power, the CPU must follow the speed for providing controls over the exposure power, a demand for the circuit configuration becomes more strict.